TRUSS SYSTEM TECHNICAL FIELD [0001] The present invention relates to building structures , more particularly, to roof truss, roof panelling an curtain wall systems. BACKGROUND [0002] The covering of outdoor and indoor areas, as well as building facades, with light transmitting materials is well known, and polymer, typically polycarbonate sheeting or panels for this purpose, are in common use. A range of materials and configurations of supporting members are used to span the area or facade required to be covered. Conventional systems rely on framing, typically, in the case of roof structures, comprising sloping rafters with regular arrays of transverse purlins, the latter providing both stabilising elements and support for the polymer sheeting. [0003] To span relatively large areas, such as sporting venues for example, requires large and heavy structural timber or steel trusses, adding considerably to the cost of the roof structure . [0004] The use of timber or steel trusses with the added array of purlins is often undesirable from an aesthetic point of view, and in some environments, such as indoor swimming pools for example, these structural elements can be subject to corrosion and deterioration. [0005] t is an object of the present invention to address or at least ameliorate some of the above disadvantages. Notes [0006] The term "comprising"'' (and grammatical variations thereof) is used in this specification in the inclusive sense of ^having" or including", and not in the exclusive sense of "consisting only of". [0007] The above discussion of the prior art in the Background of the invention / is not an admission that any information discussed therein is citable prior art or part of the common general knowledge of persons skilled in the art in any country. [0008] The term fish plate" as used in this specification indicates a plate for interconnecting two structural elements, either through interlocking elements or by means of fasteners. BRIEF DESCRIPTION OF INVENTION [0009] Accordingly, in a first broad form of the invention, there is provided a truss system for roof or facade panelling of a building; a truss of said truss system including at least one truss segment; each said truss segment comprising an first elongate element and a second elongate element; said first and second elongate elements interconnected at intervals by a series of fish-plate assemblies; said fish-plate assemblies - 3 engaging with respective extruded fish-plate channels of said first and second elongate elements. [0010] Preferably, each sai first and second elongate element of a said truss segment comprises an extruded section; said extruded section including a panel retaining channel and said extruded fish-plate channel. [0011] Preferably, a said extruded section further includes a connector plate slot; said connector blade slot adapted for friction fit insertion of a connector plate, [0012] Preferably, a said extruded section further includes a cable channel; said cable channel adapted to receive a freerunning tensioning cable. [0013] Preferably, each of said series of fish-plate assemblies comprises a pair of fish-plates arranged back to back when in use; each fish-plate of said pair of fish-plates comprising a planar plate portion provided at respective upper and lower edges with flange; said flanges projecting outwardly and normal to an outer surface of said planar plate portion, [0014] Preferably, said panel retaining channel includes panel retaining elements; said panel retaining elements formed along outer edges of said panel retaining channel; said panel retaining elements directed inwards towards a median plane of said panel retaining channel. [0015] Preferably, a said truss supports said panels by retaining flange elements of said panels in said panel J - 4 - retaining channels; said panel retaining elements locking respective said flange elements of adjacent said panels into said panel retaining channel as a snap fit. [0016] Preferably, a said truss supports said panels along said first elongate elements of said truss. [0017] Preferably, a said truss supports said panels along said second elongate element of said truss. [0018] Preferably, a said truss supports said panels along both said first and said second elongate elements of said truss . [0019] Preferably, said panels interconnect adjacent one of said trusses; said panels fulfilling the function of purlins. [0020] Preferably, said first and second elongate elements of said truss comprise aluminium extrusions. [0021] Preferably, fish-plates of said fish plate assemblies comprise stainless steel pressings. [0022] Preferably, said tensioning cable comprises stainless steel wire rope. [0023] Preferably, said panels comprise transparent or translucent polymer panels. [0024] Preferably, translucent insulating panels are retained between said panels arranged along both said first and said second elongate elements of said truss . [0025] In another broad form of th invention, there is provided a method of providing roofing or curtain walling of a building or area; said method including assembly and erection of trusses; said method including the steps of (a) assembling a required number of truss segments into said trusses; each said segment comprising a first and a second elongate element, (b) interconnecting said first an second elongate elements of each said segment with at least one fish¬ plate assembly; fish-plates of said at least one fish¬ plate assembly engaging with respective fish-plate channels of said first and second elongate elements; (c connecting abutting ones of said required number of truss segments by insertion of connector plates into connector plate slots of said abutting truss segments,